In general, all plastics have a tendency to degrade and deteriorate, particularly when exposed to air or oxygen at elevated temperatures. This combination of high temperatures and the presence of oxygen causes the oxidation of the plastic material. Upon oxidation, plastics tend to become discolored and brittle, causing them to lose much of their commercial value.
In recent years, the plastics industry has sought to develop resins which may be used for a variety of high temperature applications. Such resins which may be used at high temperatures are often referred to as "engineering resins" (see, e.g., Kirk-Othmer's Encyclopedia of Polymer Science and Engineering, Second Edition, Volume 6, pages 94-129; The Condensed Chemical Dictionary, page 821). These resins may be substituted for metals in many engineering applications, since they are capable of sustaining high loads and stresses, machinable and dimensionally stable. Such engineering resins may be used in a variety of applications, such as parts for electrical motors, automotive components, etc. Engineering resins also generally require higher processing temperatures and are used in applications wherein resistance to high temperatures over long periods of time is essential.
In order to protect these engineering resins from the deleterious effects of high temperature and oxygen, two methods have been developed in the art. The first method comprises the development of new resins which are inherently more stable to higher temperatures. The second method involves the development of polymer formulations containing additives which stabilize the resin to the effects of oxygen and high temperatures. The second method has proven to be much more cost effective than the first and consequently has generally been the subject of more attention and research.
A wide variety of stabilizing additives are conventional in the art. The effectiveness of these additives depends upon the types of resin used, the temperatures to which the resin is subjected, the duration of such exposure, etc. For example, conventional stabilizers include phenols, organic phosphites and sulfides.
The phenols and organic phosphites, although more general in their utility, have demonstrated many problems. For example, high performance and high molecular weight phenols and phosphites are quite expensive, whereas the lower cost and lower molecular weight phenols and phosphites, although more cost effective to use, are more volatile and tend to be lost by vaporization at high processing temperatures. Phenol stabilizers tend to form colored impurities at high temperatures and when exposed to light, while organic phosphite stabilizers hydrolyze readily in the presence of moisture. Moreover, most phosphite stabilizers are ineffective as long-term heat stabilizers and are used normally only as processing stabilizers, i.e., in order to protect the resin from high temperatures for a short period of time (e.g., during extrusion and injection molding). The phenols and organic phosphite stabilizers contain reactive groups, such as esters and amides which may react with moisture, particularly at high temperatures.
The known organic sulfide stabilizers are only usable in polyolefins and styrenic copolymers as long-term heat stabilizers. A long-term heat stabilizer is an additive which protects the resin only after the manufacturing steps, i.e., during its service life. The conventional organic sulfides do not protect plastics and polymerics during high temperature processing or manufacturing steps and therefore, are ineffective for use in high temperature engineering resins. Moreover, organic sulfides, especially the thiodipropionates, often decompose at high temperatures emitting foul-smelling gases and generating colored impurites.
Examples of conventional organic sulfide stabilizers are disclosed in U.S. Pat. Nos. 3,652,680 and 3,772,246, which are incorporated herein by reference. Both of these references discuss the use of cycloalkane bis(alkyl sulfides) as antioxidants in polyolefins. European Patent Application Pulbication No. 177784, published Apr. 16, 1986, also discloses the use of cycloalkane bis(alkyl sulfides) as ultraviolet light stabilizers in polyolefins. However, the problem of stabilizing high temperature engineering-type resins to the degradative effects of oxygen and high temperatures has not been heretofore addressed.
Accordingly, it can be seen that there is a need for polymeric compositions containing engineering resins which are effectively stabilized from the adverse effects of oxygen and high temperatures through the addition of protective antioxidants. Ideally, antioxidants used to protect engineering resins should stabilize the resin against the effects of both high temperatures and oxygen during processing and should also protect the finished article during use. Moreover, the antioxidants should be thermally stable, so that the composition can endure the high temperature processing which engineering resins are subjected to. Moreover, the antioxidants should be chemically inert to water and a wide variety of fillers and reinforcing agents which are conventionally compounded with such resins.
An object of the present invention is to provide a resin blend comprising engineering resins and antioxidants which will protect the resins from the deleterious effects of oxygen and high temperatures both during processing and use. A further object of this invention is to provide antioxidants which may be used to stabilize engineering resins and which themselves are stable and inert to the effects of high temperature, water and chemicals.